Telephone subscriber station sets primarily employed carbon microphones prior to the availability of practical electret microphones and semiconductor amplifiers. Modern stations sets primarily employ electret microphones which require additional amplification in the set base to duplicate the gain properties of carbon microphones. The resulting mix of these types of sets creates an undesirable situation since handsets of one type do not perform satisfactorily with set bases of the other type.
A standard handset design that automatically emulates either type of microphone has a number of benefits. One example is to allow the addition of features, such as receive amplification for the hearing impaired, transmit amplification for the weak-speaking, or both, in handsets that are portable and useable with either type set base. Another would be to reduce cost of inventory and confusion when replacing a handset.
A further understanding of the electrical characteristics of set bases and microphones used in the marketplace is useful to appreciate why, even after many years of production of each type of base, a satisfactory method of implementing a universal handset design has not been realized. Carbon microphone bases, hereinafter referred to as carbon-type bases, can be characterized as providing bias power to their handset leads from low source resistances, in the area of 100.OMEGA., and a direct current (DC) voltage source which varies from about 3 VDC to 10 VDC over its desired operating range. Bases designed to bias electret microphones, hereinafter referred to as electret-type bases, can be characterized as providing DC power to their handset leads with high source resistances, in the range of 2K.OMEGA. a voltage source which ranges from about 1.5 VDC to 10 VDC. Low source voltages are associated with long loops from the central office and multiple off-hook telephone sets. High source voltages are associated with short loops and a single off-hook telephone set. The current and voltage levels associated with these parameters present a difficulty that prior art has been unable to resolve.
When an electret microphone equivalent is connected to the base, both types of bases can provide bias voltages of the same magnitude. When a carbon microphone equivalent is presented to the base, both types of bases can provide bias currents of the same magnitude. Thus, seemingly paradoxical conditions exist when attempting to automatically emulate the appropriate microphone by using voltage or current characteristics of a base.
Most prior art uses a combination of manually operated switches, programming plug connectors, and potentiometers, to provide compatibility with different type bases, resulting in problems of inconvenience, confusion, and error in settings by untrained users. Examples of these mechanical solutions can be found in U.S. Pat. No. 4,876,712 issued to Gary T. Brint et al., on Oct. 24, 1989 and U.S. Pat. No. 4,918,726 issued to Gary K. Snyder on Apr. 17, 1990. U.S. Pat. No. 3,536,838, issued to John M. Noonan and Peter O. Schuh on Oct. 27, 1970 discloses the use of different levels of available line current to automatically adapt an amplifier to two different types of hybrid coil environments. However it lacks the capability to cope with the aforementioned overlapping voltage and current conditions.
Accordingly, a broad object of the invention is to provide an adaptive amplifier circuit having methodology to sense the type of telephone set base with which it is operating and the capability to adapt itself automatically to emulate the desired electrical characteristics of either an electret microphone or a carbon microphone. A further objective is to provide an adaptive amplifier circuit that operates from telephone set line power.